This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2001-057175, filed Mar. 1, 2001; and No. 2001-388326, filed Dec. 20, 2001, the entire contents of both of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a guest-host liquid crystal composition and a guest-host liquid crystal display device.
2. Description of the Related Art
A guest-host (GH) type liquid crystal display device (LCD) in which a dichroic dye is dissolved in a liquid crystal material has a large viewing angle and permits a bright and clear display and, thus, attracts attentions in this technical field. In order to realize a brighter color and to permit the driving at a low voltage in the GH-LCD, it is necessary for the dichroic dye to be dissolved in the liquid crystal material in a high concentration because the GH liquid crystal having a high dye concentration has a large absorption coefficient so as to make it possible to decrease the thickness of the cell required for realizing a predetermined light absorption intensity. It is also possible to enlarge color reduction aria and to increase the contrast in the same thickness of the cell by using a GH liquid crystal having a high dye concentration.
When it comes to the solubility of the dichroic dye, the solubility in the low temperature region is important. In the case of using a dichroic dye having the solubility markedly lowered under low temperatures, the dye is precipitated if the display device is exposed to a low temperature so as to make it difficult to perform the display. Further, it is effective to use a dye that is highly amorphous and permits the supersaturated state to be stable. In the general TFT display device, however, an irregularity is generated on the substrate surface so as to promote the precipitation of the dye, with the result that the supersaturated state is not stable.
If the dye is once precipitated, it is difficult for the dye to be restored even if the temperature of the display device is brought back to room temperature, because, under the environment in which a cell having a small cell gap is filled with a liquid crystal having a viscosity higher in general than that of the liquid, the diffusion rate of the molecule is low so as to make it difficult to eliminate easily the difference in the dye concentration between the dye precipitated portion and the other region.
It was customary to use anthraquinone-based dyes, which are tough and excellent in reliability, as the dichroic dye. Further, the anthraquinone-based dyes are relatively small in the half-width of the absorption spectrum, which is advantageous in obtaining a clear color display in the stacked type display device (subtractive color mixing).
However, it is difficult to dissolve the anthraquinone-based dichroic dye, which is shaped closer to a plane, in liquid crystal molecules, which are rod-shaped, in a high concentration. Concerning the problem, the present inventors have found that the anthraquinone-based dichroic dyes having a particular feature in the molecular structure exhibit an excellent solubility in the fluorine-containing liquid crystal having a rod-like structure (H. Iwanage and K. Naito, Jpn, J. Appl. Phys, 37, L356 (1998)).
In the GH-LCD, the dichroic ratio as well as the solubility of the dichroic dye greatly affects the display performance. In general, the rod-like azo-based dye is known to have a large dichroic ratio. In the case of the azo-based dye, it is reported that the order parameter of the liquid crystal is increased by the dissolution of the dye. On the other hand, where the anthraquinone-based dye having a planar structure is dissolved in a liquid crystal, the order parameter of the liquid crystal is lowered. Particularly, the tendency is rendered prominent under the state of a high dye concentration.
Under the circumstances, the present invention is intended to provide a guest-host liquid crystal composition having a high dichroic ratio and capable of maintaining a high dye concentration even under a low temperature.
Another object of the present invention is to provide a liquid crystal display device excellent in display characteristics and reliability.
According to an aspect of the present invention, there is provided a guest-host liquid crystal composition, comprising:
a host liquid crystal material, the liquid crystal material having a condensed ring in the molecular structure; and
a dichroic dye mixed as a guest in the host liquid crystal material, the dichroic dye having a condensed ring as a substituent bonded to the basic skeleton.
According to another aspect of the present invention, there is provided a liquid crystal display device, comprising:
a substrate having an electrode mounted thereto;
a liquid crystal layer arranged on the electrode; and
a transparent electrode arranged on the liquid crystal layer;
wherein the liquid crystal layer comprises a host liquid crystal material and a dichroic dye mixed as a guest in the host liquid crystal material, the liquid crystal material having a condensed ring in the molecular structure, and the dichroic dye having a condensed ring as a substituent bonded to the basic skeleton.